What having a sky marshal really means

So, just how would the emergency exits get opened when the aircraft is on
the ground and needs to be evacuated, if the cabin pressure is significantly
higher than the outside pressure?

I'm guessing that the pressure differential at ground level isn't enough to
prevent the exits from opening, but that this differential is (logically)
much higher at altitude, and thus would prevent the exits from opening in
flight. But I could be wrong.

And yes, sadly the whole newsgroup thing is a never-ending disappointment.
The most humorous recent example came from the "gun loons" thread, where I
suspect that 5 or 6 people are STILL arguing about the difference between a
"box cutter" and a "carpet knife".

"Geoffrey Barnes" wrote in message
link.net...

I'm guessing that the pressure differential at ground level isn't enough
to
prevent the exits from opening, but that this differential is (logically)
much higher at altitude, and thus would prevent the exits from opening in
flight. But I could be wrong.


The pressure differential wouldn't have to be that great to create a
considerable force holding the exit in place. I don't know what the surface
area of an exit is, but a square twenty inches on a side has an area of 400
square inches. It would take 200 pounds of force to overcome a 1/2 psi
pressure differential.

"Geoffrey Barnes" wrote

So, just how would the emergency exits get opened when the aircraft is
on the ground and needs to be evacuated, if the cabin pressure is
significantly higher than the outside pressure?

I'm sure that every aircraft has some differences, but the Boeing B-707
and B-727 that I flew worked as follows:

It is desireable to have "some" pressurization differential at takeoff
for passenger comfort. This prevents the outflow valves from slamming
shut at liftoff with the engines at takeoff thrust causing a discomforting
pressure "bump". For obvious reasons, it is also desireable to have the
cabin de-pressurize after landing.

To accomplish these two goals, the pressurization controller has a switch
labeled "takeoff" and "landing". Normally left in the "landing" position,
the outflow valves are electrically driven open. Just before takeoff, the
switch is placed in the "takeoff" position and the automatic features of
the controller closes the valves somewhat to cause no more than a .125 psi
differential. This pressure limit is an FAA aircraft certification limit
and is sometimes stated as "no more than 250 ft below field elevation". The
reason for the limit is obvious....to permit an emergency evacuation.
Sometime during flight, the controller switch is placed in the landing mode
and upon touchdown, the "squat switch" on the landing gear signals the
controller to open the outflow valves.

At a maximum of .125 psi differential, the air load on an average emergency
exit would be less than 100 lbs.

Bob Moore
ATP B-707 B-727
PanAm (retired)

Robert Moore wrote in message .6...
Just before takeoff, the
switch is placed in the "takeoff" position and the automatic features of
the controller closes the valves somewhat to cause no more than a .125 psi
differential. This pressure limit is an FAA aircraft certification limit
and is sometimes stated as "no more than 250 ft below field elevation". The

Thanks for the excellent explanation, Bob. Back when I had the
misfortune of having to fly commercially, I used to notice that my
altimeter watch would drop about 200 ft. as we taxied away from the
gate. I always wondered how that would impact getting the door open
in case of emergency.

John Galban=====N4BQ (PA28-180)